US 7598006 B1
A method and apparatus for embedded encoding of overlay data ordering in an in-situ interferometer is described. An in-situ interferometer is encoded, or augmented, with special or missing alignment attributes at desired positions. Exposing a sequence of the encoded in-situ interferometer onto a silicon wafer coated with a suitable recording media. Then measuring the alignment attributes. The encoded overlay data is processed to verify the proper order and physical location of each overlay measurement. The data is collected without increasing the overall number of required overlay measurements required. Collection of overlay data allows for the proper reconstruction of the aberrated wavefront. Non-coupling alignment attribute offsets can also be used to perform similar operations using singular value decomposition and null space operations.
1. A method of embedded encoding of overlay data ordering, the method comprising:
exposing a reticle pattern in a first position onto a substrate with a recording media, the reticle comprising a plurality of separate and distinguishable alignment attributes, wherein a desired number of the alignment attributes at desired locations are shifted with non-coupling offsets;
measuring the exposed reticle pattern;
determining non-coupled portion of the overlay data and reticle error; and
correlating known overlay data with known non-coupling offsets to determine a statistical integrity.
2. A method as defined in
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This application claims priority benefit of U.S. provisional Application Ser. No. 60/634,791 filed Dec. 8, 2004 entitled “Method and Apparatus for Embedded Encoding of Overlay Data Ordering in an In-Situ Interferometer” by Smith et al. Priority of the filing date of the prior application is hereby claimed, and the disclosure of the prior application is hereby incorporated by reference in its entirety.
The present invention relates generally to optical metrology and, more particularly, to photolithography used in semiconductor manufacturing.
2. Description of the Related Art
Over the past thirty years the microelectronics industry has experienced dramatic rapid decreases in critical dimension by moving constantly improving photolithographic imaging systems. Today, these photolithographic systems are pushed to performance limits. As the semiconductor industry rapidly approaches limits of optical lithography, new metrology techniques will be required to measure the integrity of the photolithographic exposure machines and the devices they help produce. Specifically, metrology techniques that can accurately determine the aberrations of the projection system as well as the alignment precision of the exposure machines will be a necessity. In addition, these new metrology techniques will require advances in the methods used to guarantee the integrity of the collected data.
Conventional techniques for collecting overlay data includes programming an overlay tool with a set of software instructions that instructs the overlay tool to measure the alignment attributes (such as, bar-in-bar, box-in-box, and others) in a distinct order. The labeling and identification of the overlay output data usually depends on the type of overlay tool used to measure the alignment attributes. For example, the KLA 5200 series tools (see, for example, KLA-Tencor, “KLA 5200 Overlay Brochure”) use a complicated coding system that requires a fair degree of interpretation to decode the output data. Other tools, like the BioRad Q7 (see, for example, Bio-Rad Semiconductor Systems, “Quaestor Q7 Brochure”), simply label the output data, matching each registration error to its unique field point. However, it is important to consider that most overlay tools are programmed to measure the overlay or registration error of identical target structures that are located in close proximity of one another. Most overlay tools use an optical recognition routine to identify each alignment attribute before each measurement. Oftentimes the optical recognition system can read the wrong alignment attribute or a similar looking feature in a systematic way. If one simply assumes that the overlay tool has identified the correct alignment attribute and proceeds to use the program for production measurements, the data can become corrupt. In addition, many times the alignment attributes and wafer exposure patterns are symmetric with respect to the notch of the wafer, and simple rotations of the wafer can cause a good deal of confusion when trying to verify the results visually. Therefore, in some cases, even if the output data is labeled correctly there is really no way to independently verify the integrity of the results for common machine and overlay tool programming errors.
Recently, extrinsic, or non-embedded, techniques for verifying overlay data orientation have been developed (see, for example, McArthur et al., “Method and Apparatus for Proper Ordering of Registration Data”, U.S. Pat. No. 6,833,221, Dec. 21, 2004). While these extrinsic techniques are useful they require that the user actually perform additional exposures and measurements. These additional exposures and measurements can be troublesome for semiconductor manufacturing facilities that limit excess metrology.
Thus, a need exists for improved methods and apparatus that can independently verify the integrity of an overlay set-up program and output data results before using the job deck programs for production applications. It is also desirable to minimize data collection during verification of the overlay set-up program.
Embodiments disclosed herein address the above stated need of verifying the integrity of an overlay set-up program. Aspects include embedded encoding that is inherent to a projected field point (PFP), measurements that cannot be omitted by the user, and a verification process that is carried out in software, invisible to the user, at each and every measured field point.
A method and apparatus for embedded encoding of overlay data ordering in an in-situ interferometer is described. An in-situ interferometer is encoded or augmented with missing alignment attributes, either aberration determining structures (MA) or reference structures (MO) at desired locations. Following an exposure sequence of the encoded in-situ interferometer onto a silicon wafer coated with a suitable recording media the alignment attributes are measured using a conventional overlay reader and job deck. The encoded overlay data is processed with appropriate software to verify both the proper order and physical location of each overlay measurement without increasing the overall number of required overlay measurements as required by Smith et al. “Apparatus, Method of Measurement and Method of Data Analysis for Correction of Optical System, U.S. Pat. No. 5,978,085, Nov. 2, 1999. Alternate embodiments make use of non-coupled offsets and changes in the physical size (width) of overlay alignment attributes.
In one embodiment, encoded patterns within a projected field point can be used to partially or completely verify the ordering and physical orientation of overlay data gathered for the purposes of in-situ interferometry as practiced according to see, for example, Smith et al., “In-Situ Interferometer Arrangement”, U.S. Pat. No. 6,693,390, Nov. 8, 2005, and Smith et al., “Apparatus, Method of Measurement and Method of Data Analysis for Correction of Optical System”, U.S. Pat. No. 5,828,455, Oct. 27, 1998, and U.S. Pat. No. 5,978,085, supra. The process of creating In-Situ Interferometer (ISI) reticles, including appropriately modified physical structures (modified MAs and MOs—see, for example, U.S. Pat. No. 6,833,221 supra), and using overlay data gathered to verify the physical location where the overlay data was gathered, is called embedded encoding. Embedded at each projected field point (PFP) is additional information describing relative spatial arrangement of physical structures (MAs and MOs). Extracting the additional information requires no additional overlay measurements; hence the adjective embedded. This is an improvement over extrinsic or non-embedded techniques for verifying overlay data orientation (see, for example, U.S. Pat. No. 6,833,221 supra) that do require additional overlay measurements which may be troublesome for semiconductor manufacturing facilities that limit excess metrology. Aspects of embedded encoding include that the information is inherent to the projected field point (PFP), the measurements cannot be omitted by the user, and the verification process is carried out in software, invisible to the user, at each and every measured field point.
In one embodiment of embedded encoding of overlay data ordering a reticle pattern is exposed onto a substrate with a recording media. The reticle includes a plurality of alignment attributes, wherein the plurality of alignment attributes include an array of aberration determining structures and an array plurality of reference structures. On the reticle, one or more of the alignment attributes at predetermined sites are removed. After exposure, the reticle pattern is measured. Then it is determined if measurements indicating missing alignment attributes within the exposed reticle pattern correlate to the predetermined sites of the reticle. The removed alignment attributes can be an aberration determining structure, or a reference structure, or both.
The substrate can be a semiconductor surface, a silicon wafer, a flat panel display, a reticle, a photolithographic mask, or an electronic recording media. The recording media can be a negative resist material, a positive resist material, an electronic CCD, a liquid crystal material, or an optically sensitive material. Also, the alignment attributes comprise a box-in-box structure, a bar-in-bar structure, or a grating. Additionally, the techniques can be used in a photolithographic stepper system, a photolithographic scanner system, a one dimensional scanner, a two dimensional scanner, an electronic beam imaging system, an extreme ultra-violet photolithographic tool, an EPL machine, or an image side immersion lens, or an x-ray imaging system. Examples of alignment attributes comprise a box-in-box structure, a bar-in-bar structure, or a grating. The method can also include interpolating the measurements indicating missing alignment attributes for reconstructing a lens aberration.
In another embodiment a reticle includes an array of aberration determining structures and an array of reference structures. At least one of the aberration determining structures at a predetermined location with the array of aberration structures is removed. The aberration determining structures can be an inner box structure and the reference structures comprise an outer box structure. Also, the aberration determining structures comprise an inner bar structure and the reference structures comprise an outer bar structure.
In another embodiment a reticle includes an array of aberration determining structures and an array of reference structures. At least one of the reference structures at a predetermined location with the array of reference structures is removed. The aberration determining structures can be an inner box structure and the reference structures comprise an outer box structure. Also, the aberration determining structures comprise an inner bar structure and the reference structures comprise an outer bar structure.
In another embodiment of embedded encoding of overlay data ordering a reticle pattern is exposed onto a substrate with a recording media, wherein the reticle includes a plurality of separate and distinguishable alignment attributes and a desired number of the alignment attributes at desired locations are shifted with non-coupling offsets. The exposed reticle pattern is measured. Then determining the non-coupled portion of the overlay data and reticle error and correlating known overlay data with known non-coupling offsets to determine a statistical integrity. It can then be determined if the statistical integrity is above a threshold. Also, the non-coupled offsets can be derived from shifts representing pupil decentering or telecentricity. And, the non-coupled offsets can include one or more vortices. In addition, reconstructing a wavefront, or a wafer-side telecentricity can be performed.
In another embodiment, a reticle includes an array of aberration determining structures and an array of reference structures. At least one of the reference structures at a predetermined location with the array of reference structures is offset with a non-coupled offset pattern.
In still another embodiment a reticle includes an array of aberration determining structures and an array of reference structures. At least one of the aberration determining structures at a predetermined location with the array of aberration determining structures is offset with a non-coupled offset pattern.
Another embodiment of embedded encoding of overlay data ordering includes exposing a reticle pattern onto a substrate with a recording media, the reticle comprising a plurality of alignment attributes. The plurality of alignment attributes include an array of aberration determining structures and an array plurality of reference structures wherein one or more of the alignment attributes at predetermined sites on the reticle are modified. Then measuring the exposed reticle pattern and determining if measurements indicating modified alignment attributes within the exposed reticle pattern correlate to the predetermined sites of the reticle. The modification of the alignment attribute includes changing the width of the alignment attribute. The alignment attribute that is modified can be an aberration determining structure, or a reference structure or both.
Still another embodiment of embedded encoding of overlay data ordering includes exposing a first reticle pattern in a first position onto a substrate with a recording media. The reticle includes a plurality of alignment attributes, wherein the plurality of alignment attributes that include an array of aberration determining structures and an array plurality of reference structures. One or more of the alignment attributes at predetermined sites on the reticle are removed. Then exposing a second reticle pattern onto the substrate, wherein the second exposure has been shifted in a desired amount so that aberration determining structures in the second pattern overlay references structures in the first exposure. Measuring the exposed reticle patterns and determining if measurements indicating missing alignment attributes within the exposed reticle pattern correlate to the predetermined sites of the reticle. The removed alignment attribute comprises an aberration determining structure, or a reference structure, or both.
Other features and advantages of the present invention should be apparent from the following description of exemplary embodiments, which illustrate, by way of example, aspects of the invention.
The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Embodiments described herein are “exemplary” and are not necessarily to be construed as preferred or advantageous over other embodiments.
In U.S. Pat. No. 5,828,455 supra and U.S. Pat. No. 5,978,085 supra, a method of providing in-situ measurement of lens aberration and other parameters is described. One method requires the ordered reading of box-in-box (alignment attributes) overlay targets by means of an optical overlay tool. Such tools, as developed for wafer inspection in semiconductor manufacturing facilities, entail creating a job deck program to parse, order and measure the overlay targets. Additionally, almost all job deck programs have no immediately apparent means of independently verifying if the overlay data has been read in the correct order. For example, as mentioned in U.S. Pat. No. 5,828,455 supra the results of the reconstructed wavefront depends on knowing exactly what overlay measurement is associated with what position both in terms of field point, as well as within a field point array. Since the programming of such optical overlay tools is sensitive to many parameters, such as desired measurement spacing (pitch), the feature size, the field point desired, and other such parameters, it is very difficult to determine if an overlay tool is properly programmed for such an application. This is aggravated by the fact that the reconstructed wavefront does not lend itself to an intuitive understanding of what the local overlay should look like. Thus, to provide an independent verification of the program and operation of overlay tools for applications sensitive to such kinds of errors, a means of independently determining the programming and functioning of the overlay tool has been invented. In general, as techniques requiring known ordered data reading using existing tools are developed, the following invention will become a key factor to their proper functioning.
An embodiment comprises a technique for verifying integrity of overlay data collected from projected field point, PFP. Typically, each PFP contains more than 100 alignment attributes, such as bar-in-bar alignment attributes—each of which is read with an overlay reader.
(BBX, BBY)=measured bar-in-bar offset as reported by overlay tool
(X, Y)1=position of inner torus
(X, Y)0=position of outer torus
Overlay reader measurements for a single PFP comprise a series of quadruples (BBXi, BBYi, XRi, YRi,) for i=1:NBB (NBB=number of BB targets per PFP) consisting of the overlay reading at site i (BBX1, BBYi) and the position the reading was taken at in reader coordinates (XRi, YRi). Now if the reader coordinates (XR, YR in
The foregoing technique is aimed at eliminating or at the very least minimizing the frequency of occurrence of these and other overlay ordering errors.
Step 1: Provide ISI
Flow begins in block 502. In block 502 an In-Situ interferometer (ISI,
Step 2: Expose and Develop Wafer
Flow continues, in
Step 3: Read Projected Field Points
Flow continues, in
IMi=0/1 if we could not/could successfully measure the bar-in-bar at site i.
Step 4: Correlate and Assess
Flow then continues to block 508. In block 508, software that uses the understanding of the relation between the reader coordinate system (XR, YR) and the wafer coordinate system (as provided for example by the method of U.S. Pat. No. 6,833,221 supra) can then convert the reported reader coordinate system positions (XRi, YRi) into indices (IXi, IYi) (
On the other hand, we can construct an array based on our foreknowledge of the scanner NA (and possibly obscuration ratio ε), scanner reticle side telecentricity, ISI geometry (especially aperture plate standoff distance and pinhole (PH) diameter), resist exposure conditions, and the known site locations of ‘missing teeth’ (PMB) using the triplet, (IXCi, IYCi, IMCi) (i=1:NBB) that tells us whether a particular site (IXCi, IYCi) should be measurable (IMCi=1) or immeasurable (IMCi=0). For example, in calculating IMCi we observe the following simple rules; if a bar-in-bar structure is so close to the exit pupil edge so that the calculated intensity relative to the nominal intensity (Inominal) on the BB is ˜0.2*Inominal, then we set IMCi=0. In consequence, all sites with missing features (i.e., all PMB sites) have IMCi=0 as do all sites definitely located in regions where no MA should naturally print (e.g., site 110 in
The rule then for assessing whether the data is potentially valid is to compare the two arrays, IMi and IMCi (i=1:NBB), and if for any i value we have:
However, before the software moves onto the reconstruction of the next field point wavefront, we can assess with some level of confidence possible causes so the user can then adjust the measurement program. Regarding IMC as a function of IX and IY (IMC (IX, IY)) we can carry out the test of Equation 3 using:
Step 1: Provide ISI with Non-Coupling Offsets
Flow begins in block 902. In block 902, referring to Block 3, we first provide an ISI with non-coupling offsets (NCO) on either the MAs, the MOs, or possibly both. NCO are pre-calculated offsets of the reticle MA (or MO) positions from their ideal, regularly spaced positions that do not couple to aberrations. Mathematically, if (BBXi, BBYi) are overlay measurements that reconstruct to some set of Zernike coefficients, a(j=1:NZ), then NCO offsets (BBXNCOi, BBYNCOi) are added to the overlay measurements so the input to our wavefront reconstructor is (BBXi+BBXNCOi, BBYi+BBYNCOi) and the output of our reconstructor will again be the numerically same Zernike aberration coefficients, a (j=1:NZ).
(X, Y)=position on reticle
(X0, Y0)=nominal position on reticle of center of MA
P′=nominal pitch on wafer
(IX, IY)=spatial indices
To the right of MA (
(DXNCO, DYNCO) (IX, IY)=non-coupling offsets at position IX, IY within MA and the other symbols have the same meaning. The vector pattern in the right half of
A key to NCO is their property of not contributing to the final Zernike coefficients, a (j=1:NZ), that are output by the wavefront reconstructor. The wavefront reconstruction methods of U.S. Pat. No. 6,693,390 supra, and U.S. Pat. No. 5,828,455 supra and U.S. Pat. No. 5,978,085 supra can be compactly described as one of solving a set of over determined equations for Zernike coefficients aj; the matrix-vector equation is:
D=2*NBB×NZ (row×column) design matrix (known)
a=NZ Zernike coefficient vector (unknown)
BBM=2*NBB measured bar-in-bar overlay data vector (known)
BBC=2*NBB corrections to bar-in-bar overlay data vector (known)
NZ=# of Zernike coefficients we are solving for (in range 28:81)
NBB=# of overlay data sites measured (in range 100:600)
Overdetermination amount in Equation 7 is:
DXYNCOi=NCO offsets expressed in column vector form
WK=NO=2NBB−NZ arbitrary numbers
Uik=ith row and kth column element of matrix U
Having found a huge class of NCOs we can impose further, practical constraints. First, we do not want the total, measured BB signal to exceed ˜1 μm at the wafer since this typically makes setting up the overlay job more difficult and less accurate. Aberrations, by themselves, typically produce BB displacements 2-4× smaller. Therefore the magnitude of the WKs will be restrained so, for example, (Max|BBNCO|)<0.2−0.3 μm.
Next, because the extent of the bar-in-bar array measured is set by the scanner numerical aperture (NA) (see, for example, U.S. Pat. No. 5,978,085 supra) if we encode an MA or MO with offsets determined from Equation 11 it is preferable that they be of limited spatial extent so that when operating on lower NA scanners, they are still NCO patterns.
Further, noise sources that cause rapid spatial variation in measured bar-in-bar data from site to site are typically rejected or suppressed in the reconstruction represented by Equation 10; this is because they are largely expressible in the form of Equation 11. Therefore we also desire low spatial frequency patterns for the NCOs.
To satisfy requirements 2 and 3, the first general observation is that bar-in-bar measurements representing aberrations will at least approximately be the gradient of the wavefront φ(
Flow continues, in
Step 3: Read Projected Field Points
Flow continues to block 906. In block 906, the projected field points are read with an overlay reader in accordance with U.S. Pat. No. 5,978,085 supra or U.S. Pat. No. 6,833,221 supra.
Step 4: Extract Non-Coupling Portion and Verify Overlay Data Integrity
Flow then continues to block 908. In block 908, where having encoded the reticle to include known offsets that do not couple into the aberrations, we are now in a position to use that information. The reason for encoding these pre-determined shifts onto the MA and MOs is to verify the integrity of the overlay data. The measured overlay data (BBMi, i=1:2*NBB) includes the effects of aberrations, reticle errors (possibly measured and known), noise, and the NCO offsets as:
BBAi=aberration contribution to bar-in-bar measurements
BBNi=noise contribution to bar-in-bar measurements
BBREi=reticle fabrication error to bar-in-bar measurements
BBNCOi=NCO contribution to bar-in-bar measurements
To the extent we have previously measured the reticle and know, at least partially, the reticle fabrication errors we can subtract them out to get:
⊥ denotes the portion of the relevant quantity perpendicular to the aberration contributions (i.e., the portion remaining when Equations 15 and 16 are applied), and BBNCOi=BBNCOi ⊥ by the nature of its construction. So to the extent that our encoded NCO offsets are larger than the residual noise (BBN⊥) or the unknown portion of the reticle error (BBRE−BBRE″), we can do a useful correlation of (BBM−BBRE″)⊥ (known) with BBNCO (known). This correlation score, if it is high enough, indicates fidelity in the gathered overlay data. To the extent that the score is indeterminate, and we cannot decide if the overlay data is acceptable or not, at least the NCO encoding will not adversely impact any reconstruction results.
While the second embodiment detailed NCO patterns on a local level for each field point using possibly a curl pattern (
Then, as described in the second embodiment, reconstruction can take place without interference from the embedding if the overlay reader data produces the desired output as described above (using a statistical correlation score).
Additionally, if the ISI method (see, for example, U.S. Pat. No. 5,978,085 supra) and Zernike solver is good enough to extract 80 Zernike terms and only 60 terms are required for proper wavefront reconstruction, one could use the extra 20 Zernikes for encoding by adjusting the reticle offsets of the MA or MO patterns as described above. In this case-one has an encoded wavefront with a portion of the Zernike spectrum programmed in. The wavefront (for each field point) can then be reconstructed (without subtracting-out the programmed portion of the Zernike spectrum) and the results plotted as a Zernike power spectrum.
If the overlay reader is equipped with an optical CD measurement tool it is also possible to encode an ISI reticle by modification of the physical size (width) of the alignment attribute instead of removing MA or MO structures as describe in the first embodiment. For example in the case of a bar-in-bar pattern one can simply change the bar-width, BW, (not length) of an alignment attribute substructure from say 1 um to 2 um (see BW
While construction for the second embodiment so far has been formulated to perform embedding using the ISI reticle (see, for example, U.S. Pat. No. 5,828,455 supra and U.S. Pat. No. 5,978,085 supra) for wavefront reconstruction it is also possible to perform embedding for other methodologies where the integrity of overlay data is critical for analysis and extra or separate measurements are performed at high cost. For example, methods such as those sited in see, for example, Smith et al., “Method and Apparatus for Self-Referenced Projection Lens Distortion Mapping”, U.S. Pat. No. 6,573,986, Jun. 2, 2003, Smith “Method and Apparatus for Self-Referenced Dynamic Step and Scan Intra-Field Scanning Distortion”, U.S. Pat. No. 6,906,303, Jun. 14, 2005, and Smith “Method and Apparatus for Self-Referenced Dynamic Step and Scan Intra-Field Lens Distortion”, U.S. Pat. No. 6,906,780, Jun. 14, 2005, and Smith, “Apparatus and Process for Determination of Dynamic Lens Field Curvature”, U.S. Publication No. 2005/0243309, Nov. 3, 2005 would be prime candidates since each method requires reading in overlay data in a correct sequence and reconstructing the said data to obtain a lens or scanner distortion map similar to the ISI. Here for these cases however, since reconstruction does not rely on Zernike coefficients (orthogonal systems) the method presented in the first embodiment would be preferred. Finally, other methods for verifying the integrity of the overlay reader job such as a reference array method described in U.S. Pat. No. 5,978,085 supra are also candidates for embedded encoding since again, the overlay data must be processed in an exact order with high data integrity.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.